The electric powered passenger vehicle has long been considered one of the most attractive alternatives to conventional internal combustion engine vehicles from the standpoint of overall efficiency, environmental impact and, most recently, alternative fuel capability. Many commercial enterprises and private individuals, some under the auspices of the Federal Government, have proposed various approaches to implementing an electrically powered vehicle. To date, there have been virtually no commercially successful vehicles produced on a large scale. A large number of approaches to the implementation and control of an electric vehicle are evidenced in the patent literature. Most of the approaches fall within one of three general categories of motive power source. These categories are hybrids, D.C. motor drives and induction motor drives. The first type, that is most frequently found in the patent literature, is the hybrid vehicle, comprising a small gasoline fueled internal combustion engine which mechanically drives an electrical generator which, in turn, supplies electrical energy to an AC or DC motor. With this arrangement, the gasoline engine can operate at a constant speed (at a relatively high efficiency) and achieve a substantial fuel saving compared with an engine experiencing the conventional wide range of operation. A shortcoming of many hybrids is that they are relatively heavy, requiring an electrical generator and motor as well as the gasoline engine. Additionally, the engine requires substantial amounts of volatile liquid fuel and generates exhaust emissions.
A second approach taken in the development of electric vehicles is the use of a bank of batteries which supply electrical energy to a DC motor. A variable speed motor drive circuit provides easy and versatile control of a vehicle. The principle advantage of this arrangement is that a DC motor control system requires a relatively simple power and control circuit. Unfortunately, this advantage is often more than offset by the relatively large initial cost and maintenance expenses of the motor itself. In addition, DC machinery is relatively heavy and bulky, factors which do not lend themselves well to implementation within a lightweight compact vehicle. Finally, DC motors inherently require choppers and commutators which create sparks and RF pollution which can be controlled only at additional expense.
The third, and most attractive approach from the applicant's viewpoint, is a vehicle employing a battery bank and an AC motor. AC motors are relatively lightweight, inexpensive and efficient when compared to DC motors. AC motors, with no brushes or commutators, are more rugged and reliable than their D.C. counterparts and require substantially less maintenance. Related to the power-to-weight ratio is the fact that A.C. machines can be driven at substantially greater speeds than D.C. motors. Because A.C. motors do not generate sparks, they can readily be employed in dusty, explosive and highly humid atmospheres or high altitudes. Additionally, A.C. motors can be liquid cooled if the application so requires. Although typically superior to D.C. motors in electric vehicle applications, A.C. motors often require complex control circuits which are dedicated to associated vehicle drivetrains and can be extremely bulky and expensive. To date, virtually all A.C. electric vehicles have employed multi (usually three) phase design strategies. Although three-phase machinery has many advantages as set forth hereinabove, three-phase inverter costs and complexity have proven to be extremely high. In relatively large load applications, such as that required in a passenger vehicle, appropriately sized solid state switching devices such as SCR's or transistors are often extremely expensive. In addition, three-phase inverters, by their nature, dictate a multiplicity of components, including switching devices, again increasing system cost.
The shortcomings of the prior art may be overcome by providing an electric vehicle drivetrain such as the one disclosed in co-pending application Ser. No. 385,633. This drivetrain includes a substantially fixed D.C. power source such as a battery which energizes a single-phase brushless A.C. motor which, in turn, imparts torque to at least one tractive wheel of the vehicle. The motor is characterized by a stator adapted from mechnical grounding to a relatively stationary portion of the vehicle such as its body, and a permanent magnet external rotor disposed for rotation about the stator for magnetic interaction therewith and adapted to engage the tractive wheel. An inverter provides a power input from the power source and a power output to the motor in response to switch command signals generated by a control circuit in response to an operator demand signal. This arrangement has the advantage of providing an inexpensive and simply constructed single-phase brushless permanent magnet A.C. motor traction drive for electric road vehicles. The control circuit controls the switching of the current direction through the motor winding as the permanent magnet rotor turns. A pair of sensors, in a closely-spaced angular relationship to the reluctance torque detent, detect the position of the rotor and provide signals to the control circuit which synchronizes the switching of the current direction with the rotation of the rotor.
A difficulty is encountered in the above described A.C. motor, however, when the motor is started from an at rest position. In the rest position the rotor magnets are aligned with the stator poles. This is caused by reluctance torque which is present at all times. To start the motor, the stator magnets of the motor are repulsed. The motor may begin moving in either the forward or reverse direction. One of the sensors detects the direction of movement, and if it is the wrong direction, causes the control circuit to reverse the current direction through the winding. This reverses the direction of the motor torque. Unfortunately, because the sensors must be spaced close to the reluctance torque detent for proper operation of the motor during normal operation, the motor may not develop sufficient rotational kinetic energy to overcome the inherent reluctance torque which urges the motor toward its detent position. Instead, the motor may simply oscillate in the reluctance torque detent.